Tin

Background

Tin is one of the basic chemical elements. When refined, it is a
silvery-white metal known for its resistance to corrosion and its ability
to coat other metals. It is most commonly used as a plating on the steel
sheets used to form cans for food containers. Tin is also combined with
copper to form bronze and with lead to form solder. A tin compound,
stannous fluoride, is often added to toothpaste as a source of fluoride to
prevent tooth decay.

The earliest use of tin dates to about 3500
B.C.
in what is now Turkey, where it was first mined and processed. Ancient
metalworkers learned to combine relatively soft copper with tin to form a
much harder bronze, which could be made into tools and weapons that were
more durable and stayed sharp longer. This discovery started what is known
as the Bronze Age, which lasted about 2,000 years. The superiority of
bronze tools spurred the search for other sources of tin. When extensive
tin deposits were found in England, traders brought the precious metal to
countries in the Mediterranean area, but kept the source a secret. It
wasn't until 310
B.C.
that the Greek explorer Pytheas discovered the location of the mines near
what is now Cornwall, England. Much of the impetus for the Roman invasion
of Britain in 43
A.D.
was to control the tin trade. The chemical symbol for tin, Sn, is derived
from the Latin name for the material,
stannum.

Elsewhere in the world, tin was used in ancient China and among an unknown
tribe in what is now South Africa. By about 2500-2000
B.C.
, metalworkers on the Khorat Plateau of northeast Thailand used local
sources of tin and copper to produce bronze, and by about 1600
B.C.
bronze plows were being used in what is now Vietnam. Tin was also known
and used in Mexico and Peru before the Spanish conquest in the 1500s.

The use of tin as a plating material dates to the time of the Roman
Empire, when copper vessels were coated with tin to keep them bright
looking. Tinned iron vessels appeared in central Europe, in the 1300s.
Thin sheets of iron coated with tin, called tinplate, became available in
England during the mid-1600s and were used to make metal containers. In
1810, Pierre Durand of France patented a method of preserving food in
sealed tinplate cans. Although it took many years of experimenting to
perfect this new technique, tin cans began replacing bottles for food
packaging by the mid-1800s.

In 1839, Isaac Babbitt of the United States invented an antifriction
alloy, called Babbitt metal, which consisted of tin, antimony, and copper.
It was widely used in bearings and greatly assisted the development of
high-speed machinery and transportation.

In 1952, the firm of Pilkington in England revolutionized the glassmaking
industry with the introduction of the "float glass" method
for the continuous production of sheet glass. In this method, the molten
glass floats on a bath of liquid, molten tin as it cools. This produces a
very flat glass surface without the rolling, grinding, and polishing
operations that were required prior to the introduction of this method.

Today, most of the world's tin is produced in Malaysia, Bolivia,
Indonesia, Thailand,
Australia, Nigeria, and England. There are no major tin deposits in the
United States.

Raw Materials

There are nine tin-bearing ores found naturally in the earth's
crust, but the only one that is mined to any extent is cassiterite. In
addition to the ores themselves, several other materials are often used to
process and refine tin. These include limestone, silica, and salt. Carbon,
in the form of coal or fuel oil, is also used. The presence of high
concentrations of certain chemicals in the ore may require the use of
other materials.

The Manufacturing
Process

The process of extracting tin from tin ore varies according to the source
of the ore deposit and the amount of impurities found in the ore. The tin
deposits in Bolivia and England are located deep underground and require
the use of tunnels to reach the ore. The ore in these deposits may contain
about 0.8-1.0% tin by weight. Tin deposits in Malaysia, Indonesia, and
Thailand are located in the gravel along streambeds and require the use of
dredges or pumps to reach the ore. The ore in these deposits may contain
as little as 0.015% tin by weight. Over 80% of the world's tin is
found in these low-grade gravel deposits.

Regardless of the source, each process consists of several steps in which
the unwanted materials are physically or chemically removed, and the
concentration of tin is progressively increased. Some of these steps are
conducted at the mine site, while others may be conducted at separate
facilities.

Here are the steps used to process the low-grade ore typically found in
gravel deposits in Southeast Asia:

Mining

1 When the gravel deposits are located at or below the water level in
the stream, they are brought up by a floating dredge, operating in an
artificial pond created along the streambed. The dredge excavates the
gravel using a long boom fitted either with chain-driven buckets or with
a submersed rotating cutter head and suction pipe. The gravel passes
through a series of revolving screens and shaker tables onboard the
dredge to separate the soil, sand, and stones from the tin ore. The
remaining ore is then collected and transferred ashore for further
processing.

A tin bonnet was ofien given as a tenth anniversary gift
during the 1800s.

In the 1800s, tin was an ordinary household material particularly
popular with the working class because of its low cost and bright
luster. Made of iron or steel rolled thin and dipped in molten
tin, it was easy to manipulate, cut, and solder. Tin was used for
nearly everything that copper, pewter, brass, or silver could be
used for, but generally did not last as long. Reviewing tin
catalogs from about 1870 reveals that tin was used for far more
than cookie cutters—it was used to make children's
toys, coffeepots, lunch boxes, and even gentlemen's
spitoonsl

However, it was also popularly used to produce a gift for the
tenth anniversary, called the "tin anniversary."
While not as well-known as the twenty-fifth, which requires silver
gifts, the Victorian housewife knew she might well receive a tenth
anniversary gift of tin like the tin bonnet depicted here. Shaped
in the form of a "spoon bonnet" popular about 1870,
it is likely that this piece dates to that time. Certainly, it
can't be worn, but was meant to be displayed on a shelf as
a remembrance of that anniversary. Tinsmiths provided whimsical
gifts just for this purpose. Museum collections include not only
hats but tin shoes and decorative vases that could never be used
to hold water.

Nancy
EV Bryk

When the gravel deposits are located in dry areas at or above the
water level in the

When gravel deposits are located at or below the water level,
they are brought up by a floating dredge, operating in an
artificial pond created along the streambed. When the gravel
deposits are located in dry areas at or above the water level,
they are first broken up with jets of water pumped through large
nozzles. Next, the ore enters the cleaning or dressing shed
adjacent to the mining operation.

stream, they are first broken up with jets of water pumped through
large nozzles. The resulting muddy slurry is trapped in an artificial
pond. A pump located at the lowest point in the pond pumps the slurry
up into a wooden trough, called a palong, which has a gentle downward
slope along its length. The tin ore, which is heavier than the sand
and soil in the mud, tends to sink and is trapped behind a series of
wooden slats, called riffles. Periodically the trapped ore is dumped
from the palong and is collected for further processing.

Concentrating

2 The ore enters the cleaning or dressing shed adjacent to the mining
operation. First, it passes through several vibrating screens to
separate out coarser foreign materials. It may then pass through a
classifying tank filled with water, where the ore sinks to the bottom
while the very small silt particles are carried away. It may also pass
through a floatation tank, where certain chemicals are added to make the
tin particles rise to the surface and overflow into troughs.

3 Finally the ore is dried, screened again, and passed through a
magnetic separator to remove any iron particles. The resulting tin
concentrate is now about 70-77% tin by weight and consists of almost
pure cassiterite.

Smelting

4 The tin concentrate is placed in a furnace along with carbon in the
form of either coal or fuel oil. If a tin concentrate with excess
impurities is used, limestone and sand may also be added to react with
the impurities. As the materials are heated to about 2550° F
(1400° C), the carbon reacts with the carbon dioxide in the furnace
atmosphere to form carbon monoxide. In turn the carbon monoxide reacts
with the cassiterite in the tin concentrate to form crude tin and carbon
dioxide. If limestone and sand are used, they react with any silica or
iron present in the concentrate to form a slag.

5 Because tin readily forms compounds with many materials, it often
reacts with the slag. As a result, the slag from the first furnace
contains an appreciable amount of tin and must be processed further
before it is discarded. The slag is heated in a second furnace along
with additional carbon, scrap iron, and limestone. As before, crude tin
is formed and recovered along with a certain amount of residual slag.

The tin concentrate is placed in a furnace along with carbon in the
form of either coal or fuel oil. It is heated and forms a slag along
with the crude tin. The slag and crude tin are heated several more
times to remove impurities and recover tin hardhead.

6 The residual slag from the second furnace is heated one more time to
recover any tin that has formed compounds with iron. This material is
known as the hard head. The remaining slag is discarded.

Refining

7 The crude tin from the first furnace is placed in a low-temperature
furnace along with the crude tin recovered from the slag plus the hard
head. Because tin has a melting temperature much lower than most metals,
it is possible to carefully raise the temperature of the furnace so that
only the tin melts, leaving any other metals as solids. The melted tin
runs down an inclined surface and is collected in a poling kettle, while
the other materials remain behind. This process is called liquidation
and it effectively removes much of the iron, arsenic, copper, and
antimony that may be present.

8 The molten tin in the poling kettle is agitated with steam, compressed
air, or poles of green wood. This process is called boiling. The green
wood, being moist, produces steam along with the mechanical stirring of
the poles. It was from this crude, but effective use of wood poles that
the poling kettle got its name. Most of the remaining impurities rise to
the surface to form a scum, which is removed. The refined tin is now
about 99.8% pure.

9 For applications requiring an even higher purity, the tin may be
processed further in an electrolytic refining plant. The tin is poured
into molds to form large electrical anodes, which act as the positive
terminals for the electrorefining process. Each anode is placed in an
individual tank, and a sheet of tin is placed at the opposite end of the
tank to act as the cathode, or negative terminal. The tanks are filled
with an electrically conducting solution. When an electrical current is
passed through each tank, the tin is stripped off the anode and is
deposited on the cathode. The remaining impurities, which are generally
bismuth and lead, fall out of the solution and form a slime at the
bottom of the tank.

10 The cathodes are remelted, and the refined tin is cast in iron molds
to form ingots or bars, which are then shipped to the various end users.
Lower purity tin is usually cast into ingots weighing 25-100 lb (11-45
kg). Higher purity tin is cast into smaller bars weighing about 2 lb (1
kg).

Quality Control

The processes described have been proven to consistently produce tin at
99% purity and higher. To ensure this purity, samples are analyzed at
various steps to determine whether any adjustments to the processes are
required.

The tin hardhead is further refined, until it is molded into tin
ingots.

In the United States, the purity levels for commercial grades of tin are
defined by the American Society for Testing Materials (ASTM) Standard
Classification B339. The highest grade is AAA, which contains 99.98% tin
and is used for research. Grade A, which contains 99.80% tin, is used to
form tinplate for food containers. Grades B, C, D, and E are lesser grades
ranging down to 99% purity. They are used to make general-purpose tin
alloys such as bronze and solder.

Byproducts/Waste

There are no useful byproducts produced from tin processing.

Waste products include the soil, sand, and stones that are rejected during
the mining and concentrating operations. These constitute a huge amount of
material, but their environmental impact depends on the local disposal
practices and the concentrations of other minerals that may be present.
The slag produced during the smelting and refining operations is also a
waste product. It may contain quantities of arsenic, lead, and other
materials that are potentially harmful. Tin itself has no known harmful
effects on humans or the environment.

The Future

The use of tin is expected to grow as new applications are developed.
Because tin has no known detrimental effects, it is expected to replace
other more environmentally harmful metals such as lead, mercury, and
cadmium. One new application is the formulation of tin-silver solders to
replace tinlead solders in the electronics industry. Another application
is the use of tin shot to replace lead shot in shotgun shells.

Development work is underway to create a tin-based compound for use in
refuse disposal landfill sites. This compound will interact with heavy
metals, such as lead and cadmium, to prevent rain water from carrying them
into the surrounding soil and water table.